1. Field of the Invention
The present invention relates to a powder magnetic core and a manufacturing method thereof.
2. Description of the Related Art
Recently, low consumption power and high efficiency are in demand in electronics, information, communication devices, etc. and a trend toward a low carbon society is stronger than ever. Accordingly, also in power circuits mounted on electronics, information, communication devices, etc., a reduction in energy loss and an improvement in power efficiency are in demand. In this connection, the magnetic core of a magnetic element used in a power circuit is required to be low in the core loss (magnetic core loss). When the core loss is reduced, the loss in electric power energy is smaller, and thereby, high efficiency and energy saving can be realized.
As such magnetic cores, soft ferrite cores have been broadly used from the viewpoint of inexpensiveness and low loss. Furthermore, also powder magnetic cores obtained by compression molding composite magnetic materials obtained by adding a binder such as a resin to a soft magnetic metal powder are frequently used.
Recently, as a power voltage is lowered, use of a larger current in a power circuit is promoted, thereby a current that flows into a magnetic element tends to increase. A high saturation magnetic flux density is necessary for the magnetic core of a magnetic element demanded to respond to a large current. A soft ferrite core is low in saturation magnetic flux density; accordingly, a magnetic core used in a magnetic element demanded to respond to a large current is a powder magnetic core.
Examples of metal soft magnetic powders used in the powder magnetic cores include iron-based crystalline soft magnetic alloy powders such as Fe powders and Fe—Si based alloy powders. The iron loss of the powder magnetic core is largely divided into hysteresis loss and eddy current loss. When the hysteresis loss is further desired to be less than that of the powder magnetic core formed of the Fe-based crystalline soft magnetic powder, amorphous soft magnetic alloy powder or nanocrystal soft magnetic alloy powder having nano-size micro-crystals is used.
Examples of methods for obtaining amorphous soft magnetic alloy powders or nanocrystal soft magnetic alloy powders include a method where a quenched ribbon obtained by a single roll technique etc. is mechanically pulverized, and an atomization method. According to the atomization method, a powder can be directly obtained without going through a pulverization step. However, the range of its composition is limited by the quenching speed of an atomizer. In general, the saturation magnetic flux density is lower than that of the quenched ribbon. The quenched ribbon can generally provide a material with a higher saturation magnetic flux density than the atomized powder.
To obtain a powder magnetic core with a low hysteresis loss and a high saturation magnetic flux density, technique relating to a powder magnetic core formed of an iron-based nanocrystal magnetic powder is disclosed, for example, in Patent Document 1. A soft magnetic powder having a matrix phase structure in which crystal particles with a particle size of 60 nm or less are dispersed at a volume fraction of 30% or more in the amorphous phase as well as having an amorphous layer on the surface of the matrix phase structure is compacted and thereafter, the compact is heated to manufacture a powder magnetic core having a soft magnetic powder of a microcrystal structure having a matrix phase structure where crystal particles with a particle size of 60 nm or less are dispersed at a volume fraction of 30% or more in the amorphous phase.                [Patent Document 1] Japanese Patent Laid-Open No. 2008-294411        